Building Deck Railings

In This Article
Railing Codes
Attaching Posts
Railings & Balusters                    View all DECKING Articles

Sturdy deck railings are an important safety feature for raised decks and are required by code for any deck 30 in. or more above grade. Building codes define the minimum height and strength of the railing system, as well as the size of gaps in the railing – they must be small enough that a small child cannot slip through.

On many older decks the 4×4 posts were notched  and then lagged to the rim joist with 3/8 in. or 1/2 in lag bolts or structural screws such as LedgerLoks. More conscientious builders may have used through bolts. The post-to-joist connections were often strong enough, especially with bolts. The weak link in the system, however, was the rim joist that the post was fastened to. Apply enough force to the top of railing and the rim joist would rotate. This can be addressed by reinforcing the rim joist connection to the floor framing (details, below).

On some decks, it’s possible to use  4×4 or larger posts that run continuously from the railing to the footings, supporting both the deck structure and the railing. As long as the posts are not notched, these easily meet the most stringent deck codes.

Another approach gaining in popularity is to use steel connectors to reinforce the post attachment. By using special framing connectors engineered for this specific application, you automatically get a rugged railing and a code-approved connection. And installation is pretty straightforward once and efficient once you’ve done a few decks this way.

Whatever approach you use, make sure your lags, bolts, or other hardware are suitable for today’s highly corrosive treated lumber. That means heavy hot-dipped-galvanized or polymer coatings approved for use with pressure-treated lumber.

Code compliant deck guardrail Installation

Critical railing post connections are simplified by using engineered framing hardware.

Deck Railing Code

The International Residential Code (IRC), adopted by most US cities and towns, requires 36 in. high deck railings for decks more than 30 in. above the ground. The railing must resist a load of 200 pounds applied in any direction at the top of the railing. With the standard safety factor of 2.5, that means railing and posts must be designed to resist a 500 pound push. The main concern is a push outward from people leaning against the railing.  Few older deck railings can meet this requirement, but building inspectors and professional deck builders are starting to pay more attention to this critical detail. Even if you can squeak by with a weaker railing, do you really want to?

Attaching Posts

The problem is that the railing post acts like a lever, exerting a lot of force on the connection at the bottom. The only practical way to resist this force with 4×4 wood posts is to with steel framing connectors. Both Simpson Strong Tie and USP  make connectors specifically designed for this application. The connectors fasten to a deck joist (or joist blocking) with heavy ¼ x 1½-in. wood screws and bolt through the post with a ½-in. diameter through-bolts or threaded rods.

Simpson DTT2Z for code compliant railing post attachement

Simpson’s DTT2Z connector, shown here, and USP’s similar DTB-TZ, are designed to meet guardrail safety codes.

The Simpson connectors (DTT2Z ) can be purchased prepacked with the approved screws and a washer for the bolt head in the post. The USP version (DTB-TZ)  installs with the company’s polymer-coated WS15-GC screws. Both companies also sell stainless-steel connectors and screws, designated SS. As an engineered connection, it’s always best to use the fasteners specified by the manufacturer and not to stick with the same company for connectors and screws.

While the connectors are moderately expensive at $8 to $10 each (less by the box), they install quickly and provide a rock-solid connection that is virtually impossible to achieve any other way.

The installation details differ somewhat depending on whether the post is installed inside or outside outer  joist (called a rim or band joist) and whether the rim joist runs perpendicular or parallel to the deck joists. To avoid confusion, we’ll refer to parallel rim joist as an end joist.   Special details are also required at inside and outside corners. The most common connections  for railing posts are shown below.

The most difficult part of the installation is accurately drilling two 9/16-in. holes in each post for the 1/2-inch bolts or threaded rod. Most installers use a marking jig to locate the holes. A drill press is helpful to keep the holes square to the post. Otherwise a good eye and steady hand are essential.

Outside the rim joist. This is the most common installation and the simplest if the post aligns with a floor joist (see Detail A, below). If the post is between two joists, you will need to use two connectors, one on each nearby joist, and bolt to post to the rim joist between the two connectors.

Outside the end joist.  Where the rim joist runs parallel to the joists (sometimes called an “end joist”) blocking must be added to tie into the second joist. This joist requires two connectors as shows (Detail B, below).

Simpson DTT2Z connector for post outside rim joist

CLICK TO ENLARGE                 Posts outside the rim joist are the most common and simplest to install. Courtesy of Simpson Strong Tie.

Simpson DTT2Z connector for post outside end joist

CLICK TO ENLARGE            With the post outside an end joist, two connectors and blocking tie back to the second joist. Courtesy of Simpson Strong Tie.











Simpson DTT2Z connector for posts inside rim joists

CLICK TO ENLARGE                          Inside posts need to align with joists and require single blocking at a rim joist and double blocking, plus a threaded rod, at end joists connections. Courtesy of Simpson

Inside the rim joist. Posts placed inside the rim joist need to align with a perpendicular joist. The connecting joist needs to be reinforced with blocking fastened with 24 10d nails.  (Detail A, left).

Inside the end joist. This connection requires double blocking, plus an extra long bolt or threaded rod to transfer the load to the next joist bay. Remember all hardware must be rated for use with pressure-treated lumber — either hot-dip galvanized or treated with approved coatings.



Outside corners.  With this type of steel connector, the simplest approach is to use two posts at each corner, held back far enough to fit in the connectors and bracing.  The top rails extend beyond the posts to meet at the corner.


If using steel tension ties seems like overkill, there are simpler methods for attaching guardrail posts. Two ½ in. bolts through the rim or end joist can meet the building code as long as the rim joist is adequately braced against rotating. Depending on the local code and it’s interpretation by the building inspector (e.g., the safety factor required), the detail shown below should be acceptable in many locales. If in doubt, have an engineer calculate the specific size and number of lag bolts or structural screws  to meet the local code.

Deck post detail: attachment to the rim joist with two 1/2 inch bolts can be code-compliant.

CLICK TO ENLARGE                                                                             Bolting to the deck’s rim joist can make a sturdy post connection as long as the rim joist is adequately braced against rotating. Check with your building inspector for approval before proceeding.












To meet code standards, the railings that span from post to post should be as sturdy as the posts themselves. To achieve the necessary strength, it’s best to use at least 2×6 railing, either flat or vertical. Use the longest pieces available, with a continuous length for each side of the deck,if possible. Posts should be no more than 6 to 8 feet apart, depending on local codes.

Wood railing details for decks

CLICK TO ENLARGE                                          For a sturdy top rail, use long, continuous pieces of 2×6, chamfered or sloped to shed water. Secure balusters with screws or spiral-shanked nails. Space between balusters must be less than 4 in.

In rainy climates, the top railing should be either sloped or chamfered at the edges to shed water. A drip cap below overhanging edges is also recommended (see illustration).

At inside and outside corners where two railings meet, I often use a galvanized steel angle or flat brace to support the joint .  This is cheap, but effective, insurance against the corner separating over time. I typically use similar steel hardware to reinforce where the railing connects to the main structure.

Code requirements for balusters.  For child safety, the balusters or other decorative infill must be spaced less than 4 inches apart. In code-speak, a 4-in. diameter ball should not pass through the spaces. For railings along stairs, the spaces can be a little larger, but less than 4-3/8 in.

The ballusters or other infill between the posts should be able to resist a load of 50 pounds applied over one square foot. Using the engineer’s safety factor of 2.5, the test load would be 125 pounds. To comply with code, and for common sense, you don’t want to fill between your sturdy posts and top rails with flimsy infill.

Baluster and infill options. On a site-built wood railing, ballusters can be metal, steel cable, or any sturdy material, but the most common are 2×2, nailed or screwed directly to the rim joist or attached to the bottom rail. Three common baluster layouts are shown in the illustration.

With 2x2s, choose good stock and reject any pieces with large knots or other defects that might weaken the baluster. Use stainless steel, coated, or hot-dipped galvanized nails or screws that are rate for use with today’s pressure-treated wood.  At a minimum, use either one screw or two spiral-shank nails top and bottom on each baluster.

Manufactured Railings.  Most major manufacturers of composite and plastic decking offer manufactured railing systems that can be used with their decking or as a standalone system.  provide composite decking products.  Most prefab systems go together pretty quickly and are engineered to meet the strength requirements of the building code. They also offer the promise of low maintenance, although this is less of a concern than within decking. Vertical components, such as railing systems, get less sun exposure and less wear and tear than decking, so their tend to stand up well over time. A flat 2×6 used for the top railing that is neither sloped or chamfered is, however, prone to the same type of cupping and checking that affects decking boards.




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